How can the size of the opening in a prayer garment (jilbab) be determined (whether it is small or large) to assess whether the prayer needs to be repeated, especially given that its size was half the size of a fingertip?

Scholars have differed regarding the estimation of the slight 'awrah (private parts) that is excused in prayer. The preponderant opinion is that the criterion for its determination is custom. Thus, what people of sound nature consider slight is slight, and what they consider extensive is extensive. The estimation of indecency varies according to the location of exposure. So, the measure of a fingertip might be considered slight if it is far from the grave 'awrah.